It is known that photons interact with semiconductor to generate electron-hole pairs. The carriers may be accelerated by the electric field within the depletion region of a PN junction giving rise to a photo generated current. However such photo generated electrons and hole pairs have a diffusion length over which a proportion of the electron-hole pairs recombine. The diffusion length shortens (as does the lifetime of the isolated charges) with increasing dopant concentration within the semiconductor. Therefore low dopant concentration or densities give rise to longer lifetimes and longer diffusion lengths. However the bulk resistivity of the silicon substrate also increases with reduced doping and hence there is a conflict between the desire to pass current through the semiconductor and the desire to have a long diffusion length.
Manufacturers of conventional photodetectors have addressed the diffusion length problem by using very thin semiconductor wafers. Wafer thicknesses of around 150 microns have been used. However such thicknesses make the wafer susceptible to processing and mechanical damage. Such thicknesses are also not routinely handled by semiconductor fabricators involved in the manufacture of integrated circuits and hence the cost of using such thin wafers is significantly more than the cost of using standard thickness wafers, of the type used in the manufacture of integrated circuits.
The problem of fragility exists with many sensor arrangements and hence it is desirable to use thicker semiconductor wafers where possible. Often is it desirable for sensors, such as micro-machined microphones, strain gauges or other sensors using techniques such as etching to form bridge, cantilevered or other such structures, to be formed on one side of a wafer with the electronic components being formed on another side of a wafer for protection. This gives rise to a need to provide a conductive path from the sensor, be it an optical sensor or some other sensor formed within the semiconductor wafer, to the electronic components.